<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link href="https://cdn.bootcdn.net/ajax/libs/normalize/8.0.1/normalize.min.css" rel="stylesheet">
    <title>单列布局</title>
    <style>
        .nav{
            width: 100%;
            height: 80px;
            background-color: red;
        }
        .body{
            width: 100%;
            height: 500px;
            background-color: #ccc;
        }
        .footer{
            width: 100%;
            height: 150px;
            background-color: rgb(0,0,0, 0.5);
            position: relative;
        }
        .footer .btn{
            position: absolute;
            top: 50%;
            left: 50%;
            z-index: -1;
        }
    </style>
</head>
<body>
    <nav class="nav"></nav>
    <main class="body"></main>
    <footer class="footer" >
        <button class="btn" type="button">Btn</button>
    </footer>
</body>
</html>